Some forms exhibit extinctions when planes should be present (i.e. satisfy Braggs Law) but are not due to destructive interference with another planes diffraction.

Useful for determining special conditions of symmetry in a single crystal ID for body, face centered minerals as well as ones with screw axes and glide planes ? method to see differences between space groups

6XRD analyses

Can look at minerals as single crystals or as a powder

Single Crystal ? must be careful about orienting the crystal so Braggs Law is satisfied, use several different techniques, advanced machines manipulate the sample in 3 axes (x,y,z) to catch all the peaks ? required for structural determination

Powder has many particles with planes at many different orientations ? many orientations satisfy Braggs Law, intensities and locations (2T) are characteristic of specific minerals. Technique primarily used for identification

7Powder XRD analyses

With a single crystal, alignment of planes which give strong diffraction returns is very exact requires precise alignment

With a fine powder, idea is to have crystals at a wide variety of orientations so hitting that exact alignment is possible without manipulating the sample i.e. in a powder we figure a few grains are lined up correctly

8Powder X-ray Analyses

XRD analysis of a powder is a common, quick, and relatively easy way to identify minerals.

Having a mixture of minerals can be tricky, so grains are first separated if possible (small amounts of other minerals will give other peaks, but intensities are low enough that it is not a big deal)

Do lose the ability to see the details of the structure of the mineral however as the precise alignment of the mineral giving the peak is unknown and not changeable

9Analytical Techniques for Minerals

XRD (X-ray diffraction) is one of the most powerful tools for mineral identification, structural/chemical refinement, and size determination

Microscopy Optical techniques are another very powerful tool for mineral identification, identification of physical/ chemical history of minerals/rocks, and mineral association which we will also study in detail (both lecture and lab)

10More analytical techniques

Electron microscopy look at techniques which utilize how electrons (shot through a sample of mineral) interact with minerals imaging possible to very small sizes

Exactly how light is absorbed and reflected, transmitted, or refracted changes the info and is determined by different techniques

15Light Source

Light shining on a sample can come from different places (in lab from a light, on a plane from a laser array, or from earth shining on Mars with a big laser)

Can tune these to any

wavelength or range of

wavelengths

IR image of Mars Olivine is purple 16Causes of Absorption

Molecular or atomic orbitals absorb light, kicks e- from stable to excited state

Charge transfer or radiation (color centers)

Vibrational processes a bond vibrates at a specific frequency ? only specific bonds can absorb IR though (IR active)

17Vibrational spectroscopy

Another name applied to absorption spectroscopy in IR range and for Raman spectroscopy

Sensitive to the vibrational modes of bonds between atoms rather than of the ions themselves

18Optical Spectroscopy

Techniques concerned with how light reflects, absorbs, or transmits through minerals from near UV to mid-infrared (250 3000 nm wavelengths)

Dealing with energy which excited electrons from a standard to an excited state

19Spectroscopy

Exactly how light is absorbed and reflected, transmitted, or refracted changes the info and is determined by different techniques

20Reflectance Spectroscopy

Can be optical or vibrational

Non-destructive form of analysis, used to see some of the chemistry, bonding

Spectroscopy is particularly good at detecting water and OH groups in minerals (especially in IR)

Good at differentiating between different clays because it detects OH groups well

21Raman Spectroscopy

Another kind of spectroscopy which looks at a scattering effect and what that tells us about the chemistry, oxidation state, and relative proportions of different ions

22Mössbauer Spectroscopy

Special effect, restricted to specific isotopes of certain elements which causes a very characteristic emission (after getting hit with a beam of gamma radiation) which is sensitive to the bonding environment of that isotope (only 57Co, 57Fe, 129I, 119Sn, 121Sb)

Generally used to study Fe tells us about how Fe is bonded and its

oxidation state

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